In a conventional gas laser, such as a He-Ne laser, the gaseous working medium is contained in a capillary having a tube circular cross section, as disclosed for the He-Ne laser of U.S. Pat. No. 4,429,399. In such a laser, the laser gain decreases with increase of the diameter of the discharge region. When the diameter of the discharge region is increased beyond a certain value, for example, about 2.5-5 mm for a He-Ne laser having a 1 m long discharge tube, therefore the power of the laser output will decrease. Thus, for a conventional gas laser, the normal way to increase the power of the laser output is to increase the length of the discharge region, that is, to increase the length of the discharge tube. Therefore, for a gas laser such as a He-Ne laser having high laser output power, a considerable discharge tube length must be employed, which is rather inconvenient both for operation and production.